This invention relates to ceramic compositions for use in electrooptic devices and, more particularly, to ferroelectric ceramic compositions having optical properties which are useful at millimeter wavelengths.
In the visible and near-infrared spectral region, the refractive index of materials can easily and efficiently be altered by the electrooptic or Kerr effect, most notably in ferroelectrics and liquid crystals. Examples of such materials include the so-called PLZT transparent ferroelectric ceramics in which the applications of an electric field alters the polarization state and thereby also the birefringence of the materials. Utilizing these electrooptic effects, a variety of operations can be performed on guided light beams including modulation, switching, beam deflection, frequency shifting or mixing, and the like. To date, the most popular ferroelectric material for these electrooptic applications has been lithium niobate.
At millimeter wavelengths, however, attempts to develop similar electrooptic effects in solids have met with disappointment. A basic reason for this can be explained by the lattice dynamics of solids. Optic-mode frequencies in solids are electric-field dependent, and in the visible and near-infrared spectral region, light frequencies (approximately 10.sup.15 Hz) are commensurate with the optic-mode frequencies usually found in solids. For millimeter wavelengths, however, the frequencies involved (approximately 10.sup.12 Hz) are in the range of acoustic modes in solids. These acoustic modes are unaffected by electric fields. Thus, the search for electrooptic effects at millimeter wavelengths has focused on considering materials having ultra-low frequency optic modes.
In many instances, ferroelectric transitions are characterized by a temperature-dependent optic mode whose frequency (.omega..sub.s) can be very low (approximately 10.sup.12 Hz). This soft mode frequency is proportional to (T-T.sub.o).sup.1/2 in the paraelectric phase of the material where T&gt;T.sub.o and T.sub.o is the transition temperature between paraelectric and ferroelectric phases. It is this temperature dependence that causes .omega..sub.s to achieve such small values.
However, several problems present themselves for these ferroelectric materials. On the one hand, in ferroelectric materials with T.sub.o near room temperature, the soft mode is usually over-damped by optoacoustic interactions, among other things, and thus is not electric field controllable. On the other hand, ferroelectric materials with under-damped soft modes are limited to inconveniently low temperatures. For example, the electric-field dependence of the soft-mode frequencies in KaTaO.sub.3 and SrTiO.sub.3 have been measured at low temperatures by induced Raman scattering by Fleury and Worlock. Phys. Rev. B174, 613 (1968). It was found in these materials that an electric field of approximately 10 kilovolts per centimeter (10 kV/cm) increased .omega..sub.s from 0.3 to 1.5 THz (i.e., 0.3 to 1.5 10.sup.12 Hz) at 8K. However, at temperatures above 40K, the effect of the electric field on .omega..sub.s was practically negligible. Finally, from a practical point of view, the operating temperature must be very stable so that changes in .omega..sub.s, which is temperature dependent, come about as a result of changes in the electric field rather than from temperature variations.
Accordingly, the need exists in this art for material and devices having useful electrooptical properties at millimeter wavelengths, but not at inconveniently low temperatures.